1. Field of the Invention
The present invention relates to a semiconductor device employed as a switching device.
2. Description of Related Art
In relation to a power MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), an MOSFET (SJMOSFET) having an SJ (Super Junction) structure for reducing the on-resistance and increasing the withstand voltage is known in general.
FIG. 7 is a schematic sectional view of a conventional semiconductor device including an SJMOSFET.
The semiconductor device 101 includes an N+-type (high-concentration N-type) substrate 102. An N-type drift layer 103 is laminated on the N+-type substrate 102. On the other hand, a drain electrode (not shown) is formed on a back surface (opposite to the side provided with the drift layer 103) of the N+-type substrate 102.
A plurality of P-type base regions 104 are formed on a surface layer portion of the drift layer 103 at intervals from one another. An N+-type source region 105 is selectively formed in each base region 104.
A gate oxide film 106 made of SiO2 (silicon oxide) is formed on the drift layer 103. Gate electrodes 107 are formed on the gate oxide film 106. Each gate electrode 107 is provided over base regions 104 adjacent to each other in a prescribed direction, and extends in a direction orthogonal to the prescribed direction. An interlayer dielectric film 108 made of SiO2 is laminated on the gate oxide film 106, to cover the gate electrodes 107.
A source electrode 109 is formed on the interlayer dielectric film 108. Contact holes 110 are formed on the base regions 104 to penetrate through the interlayer dielectric film 108 and the gate oxide film 106, and the source electrode 109 is connected to the base regions 104 and the source regions 105 through the contact holes 110.
P-type resurf layers 11 are formed in the drift layer 103 correspondingly to the base regions 104 respectively. Each resurf layer 111 extends from the corresponding base region 104 toward the N+-type substrate 102. Thus, portions of the drift layer 103 and the resurf layers 111 are alternately aligned, to form an SJ structure.
When applied to an inverter for a liquid crystal backlight or the like as a switching device, this SJMOSFET may not include a regenerative diode, but a parasitic diode formed by the drift layer 103 and each base region 104 may be used as a regenerative diode.
If a reverse bias is applied to the parasitic diode of the SJMOSFET, however, the drift layer 103 is so rapidly depleted that carriers stored in the drift layer 103 in an ON-state of the parasitic diode disappear at a stretch when the parasitic diode is turned off. When the parasitic diode is turned off, therefore, a reverse current flows in the parasitic diode and rapidly returns to zero (exhibits a large rate of time change), to provide the so-called hard recoverability. This hard recoverability results in occurrence of a noise (recovery noise).
In order to solve this problem, there is proposed a technique of reducing the depth of the resurf layers 11 or increasing the thickness of the drift layer 103 thereby separating bottom surfaces of the resurf layers 111 from the N+-type substrate 102, as shown by broken lines in FIG. 7 (refer to Patent Document 1, for example). According to this proposal, a depletion layer gradually extends from an interface between the bottom surface of each resurf layer 111 and the drift layer 103 between the bottom surface of the resurf layer 111 and the N+-type substrate 102 when a reverse bias is applied to the parasitic diode, whereby the reverse current flowing in the parasitic diode can softly return to zero (reverse recoverability of the parasitic diode can be approximated to soft recoverability).
However, it is known that the on-resistance of the SJMOSFET is increased as the ratio of the interval between the bottom surface of the resurf layer 111 and the N+-type substrate 102 to the thickness of the resurf layer 111 approaches 1. If an interval between the bottom surface of the resurf layer 111 and the N+-type substrate 102 is sufficiently increased in order to effectively prevent occurrence of a noise, therefore, the effect of reducing the on-resistance by the SJ structure is spoiled, and the on-resistance of the SJMOSFET is generally equalized with that of a normal MOSFET having no SJ structure.